As a representative example of a light-emitting device, a laser diode is well known. If a forward driving current is sent to the laser diode, electrons and holes are recoupled in an active layer region, so that photons are generated. The spontaneous emission light is reciprocated in the active layer by a feedback action, and laser oscillation (induced emission) results.
In general, a device for driving the laser diode is provided with an APC function for constantly maintaining the optical output or the amount of light generated, even if the characteristics of the laser diode have changed. This APC consists of a light-receiving device that receives light generated by the laser diode and generates a photocurrent corresponding to the optical output, a monitor voltage generating circuit for generating a monitor voltage with a size in which the photocurrent of the light-receiving device is multiplied by a prescribed coefficient, a differential amplifying circuit that compares the monitor voltage from the monitor voltage generating circuit with a reference voltage corresponding to a set value of the optical output generated by the laser diode and outputs an error voltage corresponding to the difference, a sample-and-hold circuit for sampling and holding the output voltage of the differential amplifying circuit, and a driving current generating circuit corresponding to the output voltage of the sample-and-hold circuit.
While the laser diode emits light in a stationary operating mode, the APC is not activated, and when the driving current supplied to the laser diode is reset, the APC is operated. In this case, a photocurrent corresponding to the optical output of the laser diode flows to the light-receiving device, and a monitor voltage corresponding to the photocurrent of the light-receiving device and the optical output of the laser diode is output from the monitor voltage generating circuit. Then, the monitor voltage is compared with the reference voltage in the error amplifying circuit, and an error voltage showing the difference or comparison error is sampled and held by the sample-and-hold circuit and given to the driving current generating circuit. The driving current generating circuit adopts the output voltage of the sample-and-hold circuit as a control voltage, generates a driving current with a current value corresponding to it, and supplies the driving current to the laser diode, and the laser diode emits light by the optical output corresponding to the current value of the driving current. If the laser diode emits light in this feedback loop and the monitor voltage reaches and converges to the reference voltage, that is, the optical output of the laser diode is equal to the set output, the feedback loop reaches an equilibrium state. In this equilibrium state, if the sample-and-hold circuit is switched from the sampling mode to the hold mode, the current value of the driving current supplied to the laser diode from the driving current generating circuit is held at the current value (reset current value) in said equilibrium state, even after switching from the APC operation to a stationary operation, so that the optical output of the laser diode is held at the set output. When the APC operation is finished, the monitor voltage generating circuit and the light-receiving device are turned off along with the mode switching of the above-mentioned sample-and-hold circuit.
In the conventional light-emitting device driving gear, when the APC operation is started, the monitor voltage generating circuit and the light-receiving device are switched from the off-state at that time to the on-state, and at the same time, the sample-and-hold circuit is switched from the current hold mode to a sampling mode.
In this case, immediately after starting the APC operation, when the monitor voltage output from the monitor voltage generating circuit raises from the level (usually, zero level) in the off-state at that time toward a reference voltage, a large error voltage is output from the error amplifying circuit, and the large error voltage is sampled by the sample-and-hold circuit and given to the driving current generating circuit. Thus, the current value of the driving current being output from the driving current generating circuit in accordance with the control voltage from the sample-and-hold circuit is increased from the set current value by that time, and the optical output of the laser diode is also changed in a rising direction from the value in the vicinity of the set output, so that the monitor output is further raised. Then, if the monitor voltage exceeds the reference voltage, the feedback loop of the APC is activated in the opposite direction, and an error voltage with a reversed polarity is output from the error amplifying circuit, so that the control voltage being given to the driving current generating circuit from the
sample-and-hold circuit is lowered, thereby reducing or dropping the current value of the driving current, the optical output of the laser diode, and the monitor voltage. Eventually, the monitor voltage output from the monitor voltage generating circuit converges to the reference voltage and reaches the equilibrium state, and the current value of the driving current is optimally set so that the optical output of the laser diode is equal to the set output.
However, as mentioned above, if the current value of the driving current is largely increased once from the set value at that time, reduced to return to the original value, and adjusted to a new set value (reset value), the time required for completion from the start of the APC operation is somewhat lengthened. In an ordinary application, compared with the speed of the characteristic change of the laser diode, the APC operation is carried out in a very short cycle, and for example, in a laser printing head, the APC operation is carried out for a line scanning. Therefore, it is usual for the set current value of the driving current to be not largely different immediately before the start of the APC operation and immediately after the completion. Nevertheless, as mentioned above, not only can it be said that a uniform large up and down or increase and decrease of the control voltage and the driving current at each cycle is a useless operation or incorrect operation, but the characteristics of the laser diode are degraded by overcurrent drive during the APC operation, or acceleration of the degradation is likely.
A general object of the present invention is to solve the problems of the above-mentioned prior art, and is to provide a light-emitting device driving gear that efficiently carries out an APC operation without providing a meaningless overcurrent.